This question came in from a reader recently, identifying a very common problem, which has several possible solutions: How come my fish-finder screen goes crazy when I’m starting my engines? When I’m cranking the engines, I see diagonal swatches of color on the screen. Once the engines are running, everything seems to be alright.
Check for Low Voltage
All marine electronic equipment, and even the engines themselves can be affected by low engine cranking voltage, which is the basic problem you are experiencing with your fish finder. All electronic equipment has a voltage operating range that must be maintained to ensure proper equipment operation. In the case of fish finders, the numbers do vary; Lowrance for example, has a recommended operating range for one of its fish finder lines that is 10-15 volts DC. When you crank your engines you are demanding a lot of electrical current to run the starter motors, and this will always bring your boat’s system voltage down, sometimes to a level that will send electronic equipment a bit haywire.
If the system voltage falls below the engineered voltage operating thresholds set by the equipment manufacturer, it’s going to act strangely. How strange? The equipment reaction will vary depending upon the equipment itself, but in the case of fish finders, the sort of bizarre screen display described above is not uncommon. Engine computers have been known to have their programming skewed due to excessively low voltage, causing engine smoke and rough running. Early GPS units were known to have all of the user-entered waypoints erased as a result of this voltage dip!
There are several strategies a boat owner can take to ensure none of these things become an issue for them. First and by far the easiest solution is to avoid turning on your equipment until after the engines are up and running!
A second strategy is to make absolutely certain your boat’s battery capacity exceeds your engine cranking current needs. You can check this with a simple voltmeter test. With your multi-meter set to the DC volts scale, connect the red lead from the meter to your battery’s positive terminal and the black meter lead (labeled COM on most meters) to your battery’s negative terminal. (See diagram.) Have your buddy crank the engine while you read the meter. You want to see a voltage reading of no less than 10.5 volts on your meter to ensure that your electronics won’t flip out. If your meter reading is lower than 10.5 V, then you either need a new battery or you need a larger battery to meet the needs of the engine starter.
Editor's Note: Send your questions for Ed Sherman to firstname.lastname@example.org.